Question

What’s Automapper for?

How will it help me with my domain and controller layers (asp.net mvc)?

Answer 1

Maybe an example will help here…

Let’s say you have a nicely-normalized database schema like this:

Orders       (OrderID, CustomerID, OrderDate)  
Customers    (CustomerID, Name)  
OrderDetails (OrderDetID, OrderID, ProductID, Qty)  
Products     (ProductID, ProductName, UnitPrice)  

And let’s say you’re using a nice O/R mapper that hands you back a well-organized domain model:

OrderDetail
+--ID
+--Order
|--+--Date
|--+--Customer
|-----+--ID
|-----+--Name
+--Product
|--+--ID
|--+--Name
|--+--UnitPrice
+--Qty

Now you’re given a requirement to display everything that’s been ordered in the last month. You want to bind this to a flat grid, so you dutifully write a flat class to bind:

public class OrderDetailDto
{
    public int ID { get; set; }
    public DateTime OrderDate { get; set; }
    public int OrderCustomerID { get; set; }
    public string OrderCustomerName { get; set; }
    public int ProductID { get; set; }
    public string ProductName { get; set; }
    public Decimal ProductUnitPrice { get; set; }
    public int Qty { get; set; }

    public Decimal TotalPrice
    {
        get { return ProductUnitPrice * Qty; }
    }
}

That was pretty painless so far, but what now? How do we turn a bunch of OrderDetails into a bunch of OrderDetailDtos for data binding?

You might put a constructor on OrderDto that takes an OrderDetail, and write a big mess of mapping code. Or you might have a static conversion class somewhere. Or, you could use AutoMapper, and write this instead:

Mapper.CreateMap<OrderDetail, OrderDetailDto>();
OrderDetailDto[] items =
    Mapper.Map<OrderDetail[], OrderDetailDto[]>(orderDetails);
GridView1.DataSource = items;

There. We’ve just taken what would otherwise have been a disgusting mess of pointless mapping code and reduced it into three lines (really just two for the actual mapping).

Does that help explain the purpose?